The present invention relates generally to a bi-directional data communications system. Various types of bi-directional data communications systems have been developed in which a single center facility and plural terminal units are interconnected via cables or radio links. Data signals are transmitted or received from the central facility to the terminal units or vice versa periodically at predetermined time intervals or at a desired time thereby carrying out bi-directional data communications. A typical example of such a bi-directional data communciation system is a community antenna television system (CATV system) which will be described with reference to FIG. 1. In this system, terminal units 28 are provided, for instance, in several ten thousands of homes 2 which are coupled to a single center 1. The center 1 and the terminal units 28 form one set or group. The center 1 is connected through coaxial cables to the terminal units 28 in the homes 2. A main cable 3 extends from the center 1. Main cable amplifiers 4 and branching units 5 are provided along the main cable 3 at predetermined points. Plural branch cables 6 extend from each branching unit 5. Extension cable amplifiers 7 and taps 8 are provided along each branch cable 6 at predetermined positions. Plural auxiliary branch cables 9 extend from each tap 8 with the auxiliary branch cables 9 leading into the homes 2. Provided in each home 2 is a terminal unit 28 including a converter 10, a television set 11 and a control box 12. The end of an auxiliary branch cable 9 is connected to the converter 10 which is in turn connected to the television set 11 and control box 12. In this fashion, the main cable from the center 1 connects the center 1 to the terminal units in the homes 2.
A signal receiving antenna 13 is installed outside the center 1. The antenna 13 is connected to a demodulator 15 in a source group 14. The source group 14 includes a video disc player 16, a video tape recorder 17, a studio 18 and other such equipment as desired. Signals from the source group 14 are applied to a modulation and transmission section 19. The section 19 includes two systems. One of the two systems includes an IF modulation circuit 20, a scrambling circuit 21, and an up-converter circuit 22 while the other includes an IF modulation circuit 23 and an up-converter circuit 24. The outputs of the up-converter circuits 22 and 24 are connected to the main cable 3. The main cable 3 is connected to a transmitter-receiver 25 which communicates with the converters 10. The transmitter-receiver 25 is connected to a computer 26 which is connected to peripheral equipment 27 such as a printer and a display unit.
The operation of the CATV system thus constructed will now be described. First, the switch of the television set is turned on and the television set is set to a predetermined designated channel which is preferably not used by local over-the-air stations. The control box 12 is then operated so that the frequency of a desired channel to be received is converted into that of the designated channel by the converter 10. The channels which can be selected by the control box can be classified into a group A channels of which television signals are received over the air and retransmitted without modification, a group B of channels of independent programs provided free of charge and a group C of channels of chargeable programs. Each group has several corresponding channels. Typically, selection can be made from about twenty to thirty channels.
For the group A, a television signal received by the signal receiving antenna 13 is demodulated by the demodulator 15 and then applied to the modulation and transmission section 19. The signal is modulated by the IF modulation circuit 23. The frequency of the signal thus modulated is increased to a predetermined value by the up-converter circuit 24. Thus, the television signal received is changed into a signal upon a desired channel which is then transmitted through the main cable 3, the branch cables 6 and the auxiliary branch cables 9 to the television sets 11.
The independent programs of group B are for instance weather forecast programs and news programs. A television signal provided by the video disc player 16 or the video tape recorder 17 or a television signal from a live program produced in the studio 18 is modulated and frequency-multiplied by the IF modulation circuit 23 and the up-converter circuit 24 into a television signal upon the desired channel which is applied to the main cable 3. The independent programs are free of charge irrespective of the number of times of television signal reception or the period of time of television signal reception. Each subscriber can receive the television signals for a basic monthly charge.
The chargeable programs of group C are typically new movie programs, special programs, or the like. A television signal provided by the video disc player 16 or the video tape recorder 17, or a television signal from a live program produced in the studio 18 is modulated by the IF modulation circuit 20. A scrambling synchronizing signal of predetermined form is added to the video signal in the television signal by the scrambling circuit 21. Because of this signal, if the television signal is received only as it is by the television set, it is impossible to reproduce normal pictures on the television set. The television signal thus treated is applied to the up-converter 22 where its frequency is increased to the frequency of a designated channel. The television signal thus processed is supplied to the main cable 3.
Upon receiving the television signal of the chargeable program by the television set in each home 2, the scrambled television signal is converted into a normal video signal by the converter 10 so that normal pictures can be observed on the television set 11. Whenever a chargeable program is received, a predetermined fee is charged to the subscriber. The sum of the monthly basic charge and the special use fee is billed to the subscriber.
In order to determine whether received programs are free of charge or not, it is necessary to detect what subscribers have used what channels for particular periods of time. For this purpose, the transmitter-receiver 25 outputs a retrieving signal at predetermined time intervals to address the converter 10 in each terminal unit with an address number assigned to that particular terminal unit to determine if a group C channel is being used at the retrieval time. In response to this, the converter 10 sends to the transmitter-receiver 25 an answering signal representative of the channel which is in use at the retrieval time. The charge data transmitted and received by the transmitter-receiver 25 is arranged and stored by the computer 26 and displayed or printed out by the peripheral equipment 27. As the retrieving signal is transmitted at fixed predetermined time intervals of typically several seconds to several tens of seconds, audience ratings can also be immediately calculated.
A subscriber can take part in programs by operating his control box 12. While watching the television set 11 the subscriber's choice or answer is encoded and sent through the coaxial cables to the center 1.
In the bi-directional data communications system as described above, a polling method is normally employed in which the center facility sends a calling signal (hereinafter referred to as "a down data signal") to each of the terminal units 28 individually and the identified terminal unit 28 responds to the down data signal by sending an answering signal (hereinafter referred to as "an up data signal") back to the center. A single cable or radio frequency or a single optical fiber is employed for transmission and reception of such data. According to the polling method, each of the terminal units 28 continuously monitors the down data signal to determine whether or not the address code identifying the terminal unit is present or not. The terminal unit responds only when the identified address code is received at which time the up data signal is transmitted. The down data signal transmitted from the center facility 1 contains various data to be transmitted to the terminal units 28 together with the address codes.
This bi-directional data communication system using a polling method is effective when the terminal units 28 are distributed over a relatively extensive area and where data communications are carried out for a large number of terminal units. If the distance between the center 1 and the terminal unit 28 is long, a relatively long time is required to transmit the data signal. That is, the time required for data communications between the center 1 and the various terminal units 28 differs depending on the distance between the center 1 and each terminal unit. Accordingly, it is impossible to receive the return up data signals at the same intervals at which the down data signals were transmitted. In addition, if a large number of terminal units 28, about several ten thousands or more, for example, are operated with a single center facility, it may be desirable to perform polling with respect only to arbitrarily selected terminal units in conformity with the kinds of the data being transmitted or the type of terminal unit. Accordingly, it is advantageous in that effective utilization of the system is possible even if a large number of the terminal units 28 are used.
On the other hand, bi-directional communications using the polling method is disadvantageous in that each terminal unit has to monitor the down data signal at all times as previously mentioned. For this reason, the construction of such a system is complicated and the cost of the system is high. However, in a relatively small system having about two to four thousand terminal units which are located in a limited area, there is very little transmission time involved in transmitting data between the center 1 and the terminal units so that insertion of the address codes into the down data signal does not lower the efficiency of the overall system.